In recent years, with the proliferation of personal computers into homes, digital still cameras (hereinafter referred to simply as digital cameras) that enable input of picture image information, such as photographed scenery, a portrait, or the like, into a personal computer are rapidly becoming more popular. Further, with the development of higher performance mobile cellular telephones, portable modular cameras for mounting in such cellular telephones and providing picture image input have also become more common.
An image pickup element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary-Metal Oxide Semiconductor), is used in these cameras. Advancements in the miniaturization of camera elements in recent years have allowed these cameras to be designed to be extremely miniaturized as a whole. Further, advancements in achieving a larger number of pixels in the same size image pickup element has led to the development of higher image resolution and higher performance.
Japanese Laid-Open Patent Application H10-48516, Japanese Laid-Open Patent Application 2002-221659, and U.S. Pat. No. 6,441,971 describe exemplary imaging lenses for such cameras that include only three lens elements. Additionally, Japanese Laid-Open Patent Application 2002-517773 describes an imaging lens that includes only four lens elements. In the imaging lens described in Japanese Laid-Open Patent Application H10-48516, the stop is positioned between the second and third lens elements, in order from the object side. In the imaging lenses described in Japanese Laid-Open Patent Applications 2002-221659 and 2002-517773, the stop is positioned between the first and second lens elements in order from the object side. In the imaging lens described in U.S. Pat. No. 6,441,971, the stop is positioned on the object side of the first lens element in order from the object side. All of the imaging lenses described in the four patent documents discussed above have a lens element with a meniscus shape at the extreme object side of the imaging lens.
Advances in miniaturization and larger numbers of pixels in the image pickup element have increased the need for imaging lenses, especially for digital cameras, with higher resolution performance and more compactness. At the same time, although cost and compactness have been the primary requirements for the imaging lens of a portable modular camera, as advancements providing larger numbers of pixels in image pickup elements continue in portable modular cameras, the demand for improvement of optical performance has also increased recently.
Accordingly, the development of a wide variety of imaging lenses based on a comprehensive consideration of cost, performance and compactness is required. The development of an imaging lens that is low in cost and which also has high performance is required, along with satisfying the requirements of compactness that enable mounting the imaging lens in, for example, a portable modular camera, as well as giving ample consideration to the optical performance for mounting the imaging lens in a digital camera.
In response to such demands, a three-lens or a four-lens construction may be used in order to satisfy the requirements of compactness and low cost, and consideration can be given to the aggressive use of aspheric surfaces in order to achieve higher performance. In this case, an aspheric surface contributes to the compactness and higher performance. However, because the use of aspheric surfaces is disadvantageous from the manufacturing point of view and it may easily increase cost, sufficient consideration should be given to manufacturing issues before a particular design using aspheric surfaces is approved for mass production.
The imaging lenses described in each of the patent documents discussed above include aspheric surfaces in their three-lens and four-lens constructions, but, for example, in order to achieve favorable optical performance, the compactness of the imaging lenses becomes unfavorable. Generally, with a lens having a three-lens construction, even if there is adequate optical performance for a portable modular camera, the performance is inadequate for use in a digital camera. Additionally, with an imaging lens having a four-lens construction, although the optical performance can be improved over a three-lens construction, the cost and size of the imaging lens increase.